1. Field of the Invention
This invention relates to a nitride semiconductor free-standing substrate comprising a nitride-based compound semiconductor crystal and a light emitting device using the substrate. In particular, this invention relates to a nitride semiconductor free-standing substrate that can provide a light emitting device, which is fabricated by using the substrate, with a small variation in emission wavelength, emission output, device lifetime etc., and that can provide the light emitting device with an enhanced yield, and relates to the light emitting device using the substrate.
2. Description of the Related Art
Nitride semiconductor materials have a sufficiently wide bandgap and are of direct transition type in inter-band transition. Therefore, they are a great deal researched to be applied to a short-wavelength light emitting device. Further, they have a high saturation drift speed of electron and can use two-dimensional electron gas obtained by hetero junction. Therefore, they are also expected to be applied to an electronic device.
Nitride semiconductor layers constituting these devices are epitaxially grown on a base substrate by a vapor phase growth method such as MOVPE (metalorganic vapor phase epitaxy), MBE (molecular beam epitaxy), HVPE (hydride vapor phase epitaxy) etc. However, it was difficult to obtain a high quality growth layer since no underlying substrate is found which lattice-matches to the nitride-based semiconductor layer. Thus, a number of crystal defects must be included in the obtained nitride semiconductor layer. The crystal defect is an impediment to the improvement of the device characteristics. Therefore, it is actively researched to reduce the crystal defect in the nitride semiconductor layers.
A known method for growing a group III nitride-based compound semiconductor crystal with a relatively low crystal defect density is conducted such that a low-temperature deposition buffer layer is formed on a hetero-substrate such as a sapphire substrate, and then an epitaxial layer is grown thereon. In the crystal growth method using the low-temperature deposition buffer layer, AlN or GaN is deposited on the substrate such as a sapphire substrate at about 500 degrees C. to form thereby an amorphous film or continuous film partially containing polycrystal. Then, the film is heated at about 1000 degrees C., and thereby the film is partially vaporized or crystallized so that high-density crystal nuclei are formed. By using it as growth nuclei, GaN film with a relatively good crystal quality is obtained. However, even when conducting the growth method using the low temperature deposition buffer layer, the resultant substrate must have a considerable number of the crystal defects such as a penetrating dislocation and a void. Therefore, this method is insufficient for producing a presently-desired high-performance device.
To solve the problem described above, in recent years, a GaN single crystal substrate (hereinafter referred to as “GaN free-standing substrate”) as a GaN substrate for growing crystal has been developed. For example, JP-A-1999-251253 discloses a method for producing the free-standing GaN substrate. The method is conducted such that a GaN layer is grown on a sapphire substrate by using ELO (epitaxial lateral overgrowth), which is a method to obtain a GaN layer with a low dislocation density by forming a mask with openings on an underlying substrate and growing laterally the GaN layer through the openings of the mask, and then the sapphire substrate is removed by etching to obtain the GaN free-standing substrate.
A further advanced method than the ELO is FIELO (facet-initiated epitaxial lateral overgrowth) (e.g. Akira Usui et. al., “Thick GaN Epitaxial Growth with Low Dislocation Density by Hydride Vapor Phase Epitaxy”, Jpn. J. Appl. Phys. Vol. 36(1997) pp. L899-L902). Although the FIELO is similar to the ELO in that a selective growth is conducted by using an oxide silicon mask, it is different than the ELO in that a facet is formed on the opening of the mask in the selective growth. By the facet thus formed, the dislocation is changed in propagation direction thereof, so that the penetrating dislocation reaching the upper face of the epitaxial growth layer can be reduced. Therefore, by growing a thick GaN film on an underlying substrate such as sapphire by using the FIELO and then removing the underlying substrate, a high-quality GaN free-standing substrate with relatively reduced crystal defects can be obtained.
Other than the above methods, DEEP (dislocation density elimination by the epi-growth with inverted-pyramidal pits) is known as a method for producing the GaN free-standing substrate with a low dislocation density (e.g. Kensaku Motoki et al., “Preparation of Large Freestanding GaN Substrates by Hydride Vapor Phase Epitaxy Using GaAs as a Starting Substrate”, Jpn. J. Appl. Phys. Vol. 40(2001) pp. L140-L143). The DEEP is conducted such that GaN is grown on a GaAs substrate by using a patterned mask of SiN etc. while intentionally forming pits surrounded by facets on the surface of crystal, and a dislocation density is accumulated at the bottom of the pits so that a low dislocation density can be obtained in the other region to the pits.
The GaN substrate obtained by the ELO and the FIELO has usually, in the as-grown crystal, morphology such as a pit and a hillock on the surface thereof. Thus, it is difficult to grow thereon an epitaxial layer for a device. Therefore, it is typically conducted that the GaN substrate is polished to have a mirror surface and is used for fabricating the device.
Further, as a method for producing the GaN substrate with a low dislocation density, JP-A-2003-178984 discloses a method that a GaN layer is formed on c-face ((0001) face) of a sapphire substrate, a titanium film is formed thereon, the substrate is heated in an atmosphere containing hydrogen gas or hydrogen-containing compound gas to form voids in the GaN layer, and a GaN semiconductor layer is formed on the GaN layer.
It is true that the dislocation density in the GaN free-standing substrate produced by the above methods is reduced. However, according to further researches of the inventor, it is found that, when plural devices are fabricated by growing a light emitting device structure on the GaN free-standing substrate and being mounted, a large variation in emission wavelength are recognized among the devices, and some of the devices have a very low emission output and lifetime. When the emission wavelength, emission output and lifetime of the devices are thus varied relative to designed values thereof, the yield of the device will lower.